Hereinafter, a known angular velocity sensor will be described with reference to the drawings.
FIG. 7 is a plan view of a detecting element which is used in a known angular velocity sensor. FIG. 8 is a sectional view of an arm section of a detecting element, which is used in the same angular velocity sensor, taken along the line 8-8.
In FIGS. 7 and 8, the known angular velocity sensor has detecting element 1 for angular velocity detection, an electronic circuit (not shown) connected to detecting element 1, and a case (not shown) housing detecting element 1 and the electronic circuit.
Detecting element 1 has a tuning fork shape in which a pair of arm sections 3 are provided in shaft section 2. Drive electrode section 4 and sensing electrode section 5 are provided to extend from arm sections 3 to shaft section 2. Monitor electrode section 6 is provided to extend from arm sections 3 near shaft section 2 to shaft section 2.
Drive electrode section 4 is an electrode for inputting a drive signal for driving detecting element 1. Monitor electrode section 6 is an electrode detecting the drive status of detecting element 1 and outputting a monitor signal. Sensing electrode section 5 is an electrode outputting a sensing signal, which is generated due to an angular velocity given to detecting element 1.
As shown in FIG. 8, drive electrode section 4, monitor electrode section 6, and sensing electrode section 5 each have lower electrode 7 formed on a tuning fork-shaped substrate, piezoelectric film 8 formed of a piezoelectric material on lower electrode 7, and upper electrode 9 formed on piezoelectric film 8. A conductive layer that becomes lower electrode 7 is formed on silicon base 10, piezoelectric film 8 is formed on the conductive layer, and a conductive layer that becomes upper electrode 9 is formed on piezoelectric film 8. The conductive layers and piezoelectric film 8 are dry etched to have a predetermined shape.
When minute detecting element 1 is formed, if wet etching is used, the conductive layer that becomes lower electrode 7 or upper electrode 9 or piezoelectric film 8 may be unduly etched by an etchant, and the characteristics may be deteriorated. For this reason, the conductive layer or piezoelectric film is treated by dry etching capable of comparatively accurately etching only a necessary section.
The operation status of the known angular velocity sensor will be described.
If a drive signal is input to drive electrode section 4, a pair of arm sections 3 are driven to vibrate. In a state where a pair of arm sections 3 are driven to vibrate, if an angular velocity is given to detecting element 1, a pair of arm sections 3 are deflected in a direction in which a Coriolis force is applied, and a sensing signal is output from sensing electrode section 5. A monitor signal that is synchronized with vibration of a pair of arm sections 3 is also output from monitor electrode section 6.
When an angular velocity is given to detecting element 1, arms sections 3 are deflected, and a sensing signal is output from sensing electrode section 5 of detecting element 1. The sensing signal is detected based on a detection signal generated from the monitor signal, which is output from monitor electrode section 6, and then an angular velocity signal is detected.
In this instance, the sensing signal is output with an angular velocity component and a noise component superimposed thereon. FIG. 9 shows characteristic waveforms of a signal processing regarding noise component filtering in a state where no angular velocity is given to detecting element 1 (in a state where no angular velocity component is generated). In respective waveform charts of FIG. 9, the vertical axis denotes voltage (V), and the horizontal axis denotes time (t).
As shown in FIG. 9, detection signal 41 is generated from monitor signal 40, and differential amplification signal 43 is generated from sensing signal 42. Phase shifter signal 44 and inverting amplification signal 45 are generated from differential amplification signal 43. Phase shifter signal 44 and inverting amplification signal 45 are detected based on detection signal 41 to generate synchronous detection signal 46. Synchronous detection signal 46 is smoothed to detect angular velocity signal 47. No angular velocity component is generated in angular velocity signal 47, and thus angular velocity signal 47 becomes “0”.
For example, Patent Document 1 is an example of related art.
In general, sensing electrode section 5 or monitor electrode section 6 has lower electrode 7 and upper electrode 9 with piezoelectric film 8 interposed therebetween. For this reason, capacitance is formed between lower electrode 7 and upper electrode 9. Due to capacitance, sensing signal 42 that is output from sensing electrode section 5 is phase-shifted, and monitor signal 40 that is output from monitor electrode section 6 is phase-shifted. As a change in temperature increases, the phase shift amount increases. Sensing signal 42 and monitor signal 40 are different in the phase shift amount with respect to the change in temperature. As the change in temperature increases, a difference in the phase shift amount between sensing signal 42 and monitor signal 40 increases.
FIG. 10 shows characteristic waveforms of a signal processing regarding noise component filtering when a difference in the phase shift amount increases. In respective waveform charts of FIG. 10, the vertical axis denotes voltage (V), and the horizontal axis denotes time (t). In FIG. 10, detection signal 51 is generated from monitor signal 50, differential amplification signal 53 is generated from sensing signal 52, and phase shifter signal 54 and inverting amplification signal 55 are generated from differential amplification signal 53. It is assumed that phase shifter signal 54 and inverting amplification signal 55 are detected based on detection signal 51 to generate synchronous detection signal 56, and synchronous detection signal 56 is smoothed to detect angular velocity signal 57.
As shown in FIG. 10, if a difference in the phase shift amount increases, sensing signal 52 is phase-shifted. For this reason, phase shifter signal 54 and inverting amplification signal 55 that are detected based on detection signal 51 are phase-shifted. As a result, appropriate synchronous detection cannot be performed. That is, although no angular velocity component is generated in angular velocity signal 57, an output voltage is generated and becomes a detection error.
As described above, according to the known configuration, sensing signal 52 and monitor signal 50 are phase-shifted or a difference in the phase shift amount between sensing signal 52 and monitor signal 50 changes depending on a change in temperature. When an angular velocity is detected from sensing signal 52 based on detection signal 51 generated from monitor signal 50, a detection error of an angular velocity is likely to occur.
[Patent Document 1] Japanese Patent Unexamined Publication No. 2002-257549